A semiconductor physical quantity sensor having a beam structure in a semiconductor substrate is manufactured by using a micro machining method. This type of sensor is disclosed in, for example, U.S. Pat. No. 6,399,516. When the sensor is manufactured, as shown in FIG. 20A, a SOI substrate is used. The SOI substrate includes a silicon substrate 200, an embedded oxide film 201 and a silicon layer 202, which are stacked in this order. The silicon layer 202 in the SOI substrate is etched so that a groove 210 as a trench is formed in the silicon layer 202. The groove 210 reaches the embedded oxide film 201. As shown in FIG. 20B, the etching process for etching the groove 210 continues to perform, so that a part of the silicon layer is etched and removed, the part of the silicon layer 202 disposed under a movable-portion-to-be-formed region. Specifically, the part of the silicon layer 202 is etched in the horizontal direction. Thus, a predetermined clearance L is formed between the silicon layer 202 and the embedded oxide film 201 so that the movable portion in a beam structure is formed. The movable portion can be movable, since the movable portion is separated from the embedded oxide film 201 by a predetermined clearance L.
The clearance L between the movable portion and the embedded oxide film 201 is controlled by an etching time. However, it is difficult to control the clearance L accurately. Specifically, as shown in FIG. 21A, the aspect ratio of the trench affects the etching rate of the silicon layer 202. Here, the aspect ratio is a ratio between a depth D of the trench and a width W of the trench. When the aspect ratio of the trench is different, the etching amount X of the silicon layer 202 in the horizontal direction is different, as shown in FIG. 21B. Therefore, the cross section of the movable portion in the beam structure does not become a rectangular shape. Thus, as shown in FIG. 21C, the thickness T of the movable portion in the beam structure becomes inhomogeneous in accordance with the pattern around the movable portion. Thus, it is difficult to control the capacitance and the like accurately, so that the device design is restricted.